Asbestos have been used as a heat-resistant sealing material, for example, since they are light in weight and have excellent heat resistance. However, use of asbestos is inhibited since it causes disorders of lungs. Therefore, instead of asbestos, ceramic fibers or the like have been used. It is thought that ceramic fibers or the like have excellent heat resistance which is equivalent to that of asbestos, and no health problem may occur as long as they are handled appropriately. However, there is a trend that a higher degree of safety is required. Under such circumstances, various bio-soluble fibers have been developed in order to realize bio-soluble fibers which do not cause or hardly causes health problems even if they are inhaled in a human body (see Patent Documents 1, 2 and 3, for example).
Most of commercially-available bio-soluble fibers have a high degree of solubility in physiological saline of pH 7.4. On the other hand, it is known that when fibers are inhaled in the lung, they are captured by macrophages. At the same time, it is known that the pH around macrophages is 4.5. Therefore, it is expected that the fibers having high solubility in physiological saline having pH of 4.5 are dissolved and decomposed in the lungs.
In addition, like asbestos, conventional inorganic fibers are secondary processed into a shaped product or an unshaped product together with various binders or additives, and are used as a joint in a heat treating apparatus, a furnace such as an industrial furnace, an incinerator or the like, a joint which fills the gap of a refractory tile, an insulating brick, a shell, and a refractory mortar, a sealing material, a packing material, an insulating material, or the like. In many cases, the inorganic fibers are exposed to high temperatures, and they are required to have heat resistance.
Further, in many cases, alumina is used in member of a furnace. There was a problem that fibers contained in a secondary-processed product react with the alumina, thereby causing the secondary product or the member to adhere and melt.